The present invention relates to an electrostatic copying machine.
Electrostatic copying machines known in the art generally comprise a rotating photoconductive drum and a transparent platen for holding an original document for copying. An optical system focusses a light image of the document onto the drum to form an electrostatic image through localized photoconduction. The electrostatic image is developed by a toner substance to form a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
Usually, either the optical system or the platen is maintained stationary and the other is moved relative thereto for scanning the document. The scanning operation requires reciprocation of either the optical system or the platen. Even if the return speed of the movable scan member is made faster than the scan speed thereof, the return portion of the reciprocation constitutes wasted time. Such a configuration is inefficient for copying a large number of documents in the form of sheets.
Another drawback of such a system is that a presser plate is required to press a sheet document against the platen. The presser plate is heavy and must be raised and lowered in order to copy each document. Such an operation is tiring and unnecessarily time consuming, resulting in a copying speed which is far less than a maximum attainable copying speed. Another drawback is that the length of an original document which can be copied is limited to the length of the platen.
Although original documents come in various sizes, it is often desirable to copy the documents on copy paper of only one size, and furthermore have the entire copy sheet filled by the image. This requires that means be provided for varying the copying magnification. Although the light image size may be adjusted through variation of the position of a converging lens of the optical system and the length of the optical path of the image, it is also necessary to vary the scan speed. Where reduced size copies are to be made, the scan speed must be increased. A large drive motor is required to reciprocate a heavy platen or optical system at increased speed, and the movement is often unstable resulting in blurred copies. Increasing the weight of the platen or optical system to attain stable movement is counterproductive in that the size, cost and power requirements of the copying machine must be disproportionately increased.